It is a well known fact that lithium ion secondary batteries, which have high energy density and whose discharge capacity does not significantly decrease, have been used for a power source for mobile tools such as mobile phones and laptop computers. In recent years, with the miniaturization of mobile tools, there also is a demand for the miniaturization of lithium ion secondary batteries to be mounted therein. In addition, with the development of hybrid cars, solar power generation, and other technologies as a measure to prevent global warming, etc., the application of supercapacitors having high energy density, such as electrical double-layer capacitors, redox capacitors, and lithium ion capacitors, has been increasingly expanding, and there is a demand for a further increase in their energy density.
An electrical storage device, such as the lithium ion secondary battery or the supercapacitor, has a structure in which, for example, a positive electrode, a negative electrode, and a separator made of a polyolefin or the like between them are arranged in an organic electrolytic solution containing a fluorine-containing compound, such as LiPF6 or NR4.BF4 (R is an alkyl group), as an electrolyte. Generally, the positive electrode includes a positive electrode active material, such as LiCoO2 (lithium cobalt oxide), LiMn2O4 (lithium manganese oxide), LiFePO4 (lithium iron phosphate), or active carbon, and a positive electrode current collector, while the negative electrode includes a negative electrode active material, such as graphite or active carbon, and a negative electrode current collector, and, with respect to the shape, the electrodes are each obtained by applying the active material to the surface of the current collector and forming the same into a sheet. The electrodes are each subjected to high voltage and also immersed in the highly corrosive organic electrolytic solution that contains a fluorine-containing compound. Accordingly, materials for the positive electrode current collector, in particular, are required to have excellent electrical conductivity together with excellent corrosion resistance. Under such circumstances, currently, aluminum, which is a good electrical conductor and forms a passive film on the surface to offer excellent corrosion resistance, is almost 100% used as the material for a positive electrode current collector (as materials for the negative electrode current collector, copper, nickel, or the like can be mentioned).
One method for providing the electrical storage device with smaller size and higher energy density is to thin a current collector that constitutes a sheet-shaped electrode. Currently, an aluminum foil having a thickness of about 15 to 20 μm produced by rolling is generally used as the positive electrode current collector. Therefore, the object can be achieved by further reducing the thickness of such an aluminum foil. However, it is difficult to further reduce the foil thickness by rolling on an industrial production scale.
A possible aluminum foil production method to replace rolling is a method that produces an aluminum foil by electrolysis. The production of a metal foil by electrolysis is performed, for example, by forming a metal film on the surface of a substrate such as a stainless steel plate by electroplating, followed by the separation of the film from the substrate. Such production is well known as a method for producing a copper foil, for example. However, aluminum is an electrochemically base metal, and thus electroplating is extremely difficult. Therefore, it is not easy to produce an aluminum foil by electrolysis. Patent Document 1 discloses, as a method for producing an aluminum foil by electrolysis, a method that uses an electrolytic bath containing 50 to 75 mol % aluminum chloride and 25 to 50 mol % an alkylpyridinium chloride or an electrolytic bath prepared by adding an organic solvent to such a bath. However, in these methods, the plating solution has an extremely high chlorine concentration. This leads to a problem in that during a plating treatment, chlorine contained in the plating solution reacts with moisture in the ambient to produce hydrogen chloride gas, causing the corrosion of the equipment. Therefore, it is necessary to take a measure to prevent the production of hydrogen chloride gas or a measure to protect the equipment from corrosion due to the produced hydrogen chloride gas. In addition, the method described in Patent Document 1 also has a problem in that the applicable current density is at most about 2 A/dm2, and thus the film formation rate is low (a further increase in the applied current density causes the decomposition of the plating solution, etc., making it impossible to stably perform a plating treatment). The addition of an organic solvent, such as benzene or toluene, to the plating solution is expected to improve the film formation rate. However, these organic solvents have high toxicity and high inflammability. Because of such risks, it must be said that the addition of such organic solvents to a plating solution is problematic in terms of the ease of liquid waste disposal and safety.
In addition, even when a thin aluminum foil can be obtained by electrolysis, to be thin is not the only thing required for an aluminum foil for use as a positive electrode current collector. During the electrode manufacturing process, the foil is tensioned. In addition, during the drying process after the application of a positive electrode active material to the surface of the foil, the foil is exposed to a high temperature of 100° C. or more. Therefore, an aluminum foil for use as a positive electrode current collector has to have excellent tensile strength, and the strength has to be stably maintained even after exposure to high temperatures. Further, in order for an aluminum foil to have excellent electrical conductivity, the purity is preferably as high as possible.